The 23-year-old initially moved to Huish Park on loan from Carlisle United in October, before making the switch permanent in January.
He has been in fine form this season, netting 22 times for the Glovers to put himself on course to finish as the top scorer in League One.
"I don't think anybody thought he would do as well as he has," Johnson told BBC Sport.
"When you get a striker who scores 22 in 35 games then you've found someone special. He's been fantastic."
Yeovil travel to Bury on Saturday afternoon.